1. Field of the Invention
The present invention relates to an IC card with a power cell, such as, a memory card used for a personal computer.
2. Description of the Related Art
The configuration of a known IC card with a cell will be described in reference to FIG. 11. FIG. 11 shows the circuitry of a known IC card.
In FIG. 11, the known IC card 100 comprises a connector 1 to be connected to a system such as a personal computer (not show), a diode 2 for preventing outflow of current from a cell 8, a diode 3 for protecting the cell 8 from being charged by a power supply in the system over a power line 9, a volatile static random access memory (SRAM) 4 for storing data or the like, a large-capacity capacitor 5 for temporarily retaining a supply voltage 14 to be fed to the SRAM 4 when the cell 8 is removed, a contact 7 in the IC card to which the cell 8 is linked, and a 3-V lithium power cell 8 stowed in a cell holder.
In FIG. 11, the SRAM 4 is connected to the connector 1 over a data line 70, an address line 11, and a control signal line 12 for use in controlling the chip of the SRAM 4, and writing and reading. Reference numeral denotes a ground terminal. Reference numeral 15 denotes pull-up resistor.
Referring to FIG. 12, the operation of the known IC card will be described. FIG. 12 is a characteristic diagram showing the relationship between time and supply voltage for the SRAM 4.
Describe below is the operation for removing and replacing the dell 8 with the IC card 100 not connected to an external system. Normally, when the IC card 100 is connected to the system, the system monitors the voltage at the cell 8 in the IC card 100. When the voltage decreases to a given value (for example, about 2.5 V), the system outputs an alarm to a video screen or the like.
In response to the alarm, when the lithium cell 8 is removed, i.e., demounted for replacement, the supply voltage 14 (V.alpha.) for the SRAM 4 is discharged according to the following formula 1: EQU V.alpha.=V.sub.b e.sup.-t/cr ( 1)
where Vb (volt) denotes the initial voltage of the cell 8, C (farad) denotes the capacitance of the capacitor 5, R (ohm) denotes the internal equivalent resistance of the SRAM 4, and t (second) denotes elapsed time. Herein, the forward voltage at the diode 3 is 0 V.
The formula is plotted as a curve V.alpha. in FIG. 12. Assuming that the minimum voltage required for the SRAM 4 to retain data is Vm (volt), in order to avoid loss of stored data, an IC card user must replace the lithium cell 8 with a new one by the time instant t1 at which the supply voltage 14 for the SRAM 4 that initially equals to the voltage Vb reaches the voltage Vm.
For example, after the alarm is given, if the IC card remains unused for a prolonged period of time, the voltage Vb may equal the voltage Vm with the passage of zero time t. The time interval t1 may therefore be 0 seconds at worst.
The aforesaid known IC card has such a problem in that the time available for replacing a cell is very short or nil, at worst.